1. Field of the Invention
The present invention relates to an ultasonic method and apparatus for tissue characterization and imaging of a quantity relating nonlinear parameter B/A. The nonlinear effect arises from characteristics resulting from pressure dependence of sound velocity. The present invention is expected to be sensitive to changes in the detailed structures and properties of tissue and useful in early diagnosis of disease, such as cancer.
2. Prior Art
Ichida et al., Real Time Nonlinear Parameter Tomography in Proceeding for 43rd Convention of the Japan Society of Ultrasonics in Medicine, p. 519 (1983) discloses a method in which the changes in the velocity of sound are measured by applying pump waves. In this method, an impulsive relatively high power pumping wave is applied from perpendicular direction to the continuous low intensity probing wave of high frequency so that the phase of the probing wave is modulated instantly by the product of the nonlinear parameter B/A and the pressure of the pumping wave. The resulting modulated probing wave is detected and demodulated to derive the distribution of nonlinear parameter B/A along the probing beam. The processes are repeated by shifting the probing beam to obtain a two-dimensional image. This method, however, is limited to a small or projecting tissue because the pumping wave must be applied perpendicular to the probing wave. It is impossible to examine an internal tissue like a liver.
Akiyama et al., A Proposal: Nonlinear Parameter Imaging Method by Pulse Echo System, in Proceedings for 43 rd Convention in Japan Society of Ultasonics in Medicine, p. 521 (1983), discloses a method in which the second harmonic component generated by nonlinear effects is measured. In this method, the pulse echo signals and their second harmonics are detected. The pressure of ultrasonic waves causes the ultrasonic waves to be modulated. Therefore, their second harmonics indicate the nonlinear parameter B/A. Since the transmission frequency is different from the receiving frequency, obtaining exact data by this method without the correction of the attenuation of the sound field is difficult.